If the blood is frozen, let it thaw overnight in refrigerator. Whisk together the blood and the milk. Add the molasses, salt and pepper (and the marjoram). Gradually whisk in the flour and stir until the mixture is smooth.

Cover the batter and let it rest for at least an hour, so that the flour will swell. Meanwhile, finely mince the onion and sauté it in a little butter until softened and translucent. Add the onion into the batter.

Heat a pancake pan until very hot. Generously butter the pan and pour a first batch of the batter in the rounds. Fry until the bubbles on the batter surface begin to set and flip the pancakes over.
Continue frying with the rest of the batter. Serve the blood pancakes hot, accompanied by lingonberry jam or sugared lingonberries.

*)In Finland, pork blood is usually sold frozen, and it is available in well-equipped grocery stores.